This invention relates to voltage controlled oscillator (“VCO”) circuitry and to methods of operating such circuitry.
Frequently desired attributes of VCO circuitry include (1) ability to operate over a wide range of frequencies (sometimes including frequencies that are quite high (e.g., in the gigahertz range)), (2) low phase noise (“jitter”) at all operating frequencies, (3) low power consumption, and (4) small area requirement on an integrated circuit. To limit a VCO's power consumption, it is typical to operate it at a frequency equal to the desired output frequency (no frequency division of the VCO signal) or at half the desired frequency when quadrature outputs are used for half-rate architectures. Again, this preference to operate at the lowest frequency possible is justified by a desire to limit the oscillator's power consumption. However, low frequency operation may inconsistent with the above-mentioned possible objective of small integrated circuit area being required for the VCO.
Known VCO circuits that employ LC (inductor/capacitor) tank circuits tend to have relatively low phase noise, but are operable over only quite narrow frequency ranges. LC tank circuits that operate at relatively low frequencies are especially large users of area on an integrated circuit. Known VCO circuits that employ ring oscillators may be operable over somewhat wider frequency ranges. However, these VCOs tend to have relatively high phase noise.